1. Field of the Invention
The present invention relates to control device with current control function to protect an induction motor and drive devices, such as inverters, from overcurrent.
2. Description of the Prior Art
FIG. 1 shows a control device of an induction motor in the prior art, for example, disclosed in Japanese patent application laid-open No. 77398/1988. In FIG. 1, numeral 2 designates an induction motor, numerals 3u, 3w designate current detectors for detecting primary current of the induction motor 2, numeral 20 designates a primary frequency command generator, numeral 22 designates a converter circuit for converting three-phase alternating current into direct current, numeral 23 designates a capacitor for smoothing direct current, numeral 24 designates an inverter circuit for converting direct current into alternating current with variable voltage and variable frequency, numeral 25 designates a gate circuit for driving a main switching element in the inverter circuit 24, and numeral 30 designates a control circuit.
The control circuit 30 comprises a microcomputer 31 for carrying out control, an amplifier circuit 32 for amplifying a switching signal outputted from the microcomputer 31 and outputting the amplified signal to the gate circuit 2, a current detecting circuit 33 for making a primary current I.sub.1v [I.sub.1v =-(I.sub.1u +I.sub.w)] of residual one phase from primary currents I.sub.1u, I.sub.1w of two phases and generating absolute values I.sub.u, I.sub.v, I.sub.w of each signal, and a sample-hold circuit 34 for holding the absolute value signals I.sub.u, I.sub.v, I.sub.w outputted from the current detecting circuit 33 according to sample signals S.sub.u, S.sub.v, S.sub.w outputted from the microcomputer 31, where the hold value is a peak value of active component of the primary current.
The control circuit 30 also comprises an effective component current detecting circuit 35, a comparator 36, an overload current setting device 37, a soft start/stop circuit 38 for generating ramp function with a previously set time corresponding to a primary frequency command value outputted from the primary frequency command generator 20, and an oscillator 39 for generating pulse train proportional to output of the soft start/stop circuit 38, where output signal B of the oscillator 39 determines output frequency of the inverter circuit 24 and output A of the soft start/stop circuit 38 determines output voltage of the inverter circuit 24.
Next, based on FIG. 2, principle will be explained regarding that specific time point of primary current is sampled thereby active component of the primary current can be detected. In FIG. 2, V.sub.1u is primary voltage in u-phase, and I.sub.up, I.sub.uq are effective component and reactive component of primary current I.sub.1u in u-phase respectively. .phi. is power factor angle.
In the steady state, equation (1) applies. ##EQU1## where V.sub.1u =V.sub.1 sin.omega..sub.1 t.
That is, the effective component I.sub.up of the primary current I.sub.1u is in the same phase as that of the primary voltage V.sub.1u, and the reactive component I.sub.up has phase lagging by 90 degrees with respect to the primary voltage V.sub.1u. Accordingly, it is seen that if absolute value signal of I.sub.1u is sampled at the time points of 90 degrees and 270 degrees with respect to the phase of V.sub.1u, peak value of absolute value of the effective component I.sub.up can be detected.
Also since the primary voltage V.sub.1v in v-phase has phase lagging by 120 degrees with respect to V.sub.1u, if absolute value signal of I.sub.1v is sampled at the time points of 30 degrees and 210 degrees with respect to the phase of V.sub.1u, peak value of absolute value of the effective component I.sub.vp can be detected, and since the primary voltage V.sub.1w in w-phase has phase leading by 120 degrees with respect to V.sub.1u, if absolute value signal of I.sub.1w is sampled at the time points of 10 degrees and 330 degrees with respect to the phase of V.sub.1u, peak value of absolute value of the effective component I.sub.wp can be detected.
Next, operation of the prior art will be described.
The microcomputer 31 inputs output signal a of the soft start/stop circuit 38 and output signal b of the oscillator 39, and calculates command values V.sub.lu *, V.sub.1v *, V.sub.1w * of primary voltages to be outputted by the inverter circuit 24, and forms and outputs switching signals based on the command values. Then according to the detection principle as above described, sample signals S.sub.u, S.sub.v, S.sub.w are outputted to detect peak values of absolute values of effective components of primary currents with respect to the phase of V.sub.1u *.
Next, the sample-hold circuit 34 holds absolute values of primary currents outputted from the current detecting circuit 33 synchronizing with the sample signals S.sub.u, S.sub.v, S.sub.w Subsequently, the peak value of the absolute value of the effective current outputted through the effective current detecting circuit 35 is compared with the overload current set value by the comparator 36. As a result, if the peak value of the absolute value of the effective current is larger, control signal to reduce time variation rate of ramp function outputted from the soft start/stop circuit 38 only at the power running is outputted to the soft start/stop circuit 38.
Then, variation of amplitude and frequency of the command values V.sub.1u *, V.sub.1v *, V.sub.1w * of the primary voltages calculated in the inside of the microcomputer 31 becomes gentle, and as a result, increase of the peak value of the absolute value of the effective current is suppressed.
A control device of an induction motor in the prior art as above described is constituted in that peak value of absolute value of effective component of the primary current is detected, and time variation rate of ramp function outputted from the soft start/stop circuit 38 is reduced so that the peak value does not exceed the previously set limit value.
At the steady state where power factor angle does not vary, the effective component of the primary current can be detected exactly, but at the transient state where power factor angle varies during one period of the primary voltage, as it cannot be detected exactly, current limitation is difficult in the case that the induction motor is accelerated rapidly.
Also in the control device of the induction motor in the prior art, since current limitation is effected only at the power running state, during regenerative period until stopping the induction motor in rotating state, the current limitation cannot be effected if any measure is not done.
Further, in order to protect the inverter circuit 24 from overcurrent, the peak value of the primary current must be limited, but if reactive component is included much in the primary current, limitation of the peak value of the primary current is difficult only by limiting the peak value of the effective component.